1. Field of the Invention
The present invention relates to an inductively coupled plasma system used in Chemical Vapor Deposition (CVD).
2. Description of the Related Art
Plasma processing of semiconductor work pieces has several advantages, such as low process temperatures and high effectiveness. For example, deposition of SiO2 can be performed at temperatures below 200xc2x0 C. with a deposition rate of about 100-500 nm/min using an O2 plasma source and SiH4 gas. However, in order to perform deposition on large areas, such as on wafers having a diameter up to 300 mm (11.8 inches), with high uniformity of the coating, the plasma source must have very high productivity and form a plasma having a uniform flux. These requirements are satisfied by several kinds of High Density Plasma sources (HDP), which are available in the art.
Although historically the development of HDP started from Electron Cyclotron Resonance (ECR) plasma, most recent applications are based on Radio Frequency (RF) driven Inductive Coupled Plasma (ICP). ICP sources are simple in design, have a wide power and pressure window, and do not require auxiliary magnets for their operation. A flat spiral coil inductor is typically used for high efficiency for deposition and etching. Disadvantageously, there are some inherent drawbacks with this kind of ICP source. For example, RF power from a coil to a process chamber must be fed through a dielectric window, which is typically made of quartz. The thickness of this window must be sufficient to withstand atmospheric pressure. Typically, this thickness must be several centimeters, and for large-scale equipment, the window must be still thicker. Also, the vacuum side of the dielectric window suffers from sputtering due to high voltages on the coil along with significant capacitive coupling.
A helical resonator is a kind of ICP sources that operates under resonance conditions of a helical inductor coil. Resonance is achieved by adjusting the length L of an inductor wire to a wavelength xcex that is associated with a RF electromagnetic field exciting the discharge and is governed by the equation L=(xcex/4)*m, where m is an integer. Different m values correspond to different modes of standing waves in the inductor. Whether ends of the coil are electrically grounded or floating (they may be either) determines the different boundary conditions for current and voltage waveform. An RF tap position is usually intermediate, i.e. situated between the coil ends. By varying the boundary conditions and the inductor wire length, the plasma source may be balanced for parasitic capacitive coupling effects. This plasma source has a cylindrical geometry and must have a dielectric enclosure-plasma-containing vessel. This vessel is usually a cylindrical quartz tube (reactor), which at the same time forms the sidewall of a source vacuum chamber.
Balancing of the inductor minimizes plasma potential relative to a grounded surface and hence sputtering of a reactor material. This kind of inductor shows very high effectiveness and radial uniformity when applied for dry etching under pressure of 1 Torr or higher. However, when the pressure is below 10 mTorr, HDP with cylindrical geometry may lose radial uniformity of gas flow due to an effect of ion pumping that leads to a depletion of neutral species in an axial region. This effect is more pronounced at low pressures and high density of plasma, i.e., high concentration of charged particles. The most significant changes in neutral uniformity can occur in large-area plasma sources.
In most applications using ICP sources, an inductor is located outside a vacuum chamber. However, positioning of the inductor outside the chamber has several disadvantages. One, it requires large, complex dielectric vacuum vessels for a helical inductor or large area dielectric ports in a case of a flat spiral inductor. Two, an external inductor is not compatible with an Ultra High Vacuum (UHV) design.
In addition, while it is desirable to have the surface area of a susceptor much smaller than the surface area of the grounded portion in order to control a negative bias voltage on a substrate without applying high RF power, the ratio of the conductive portion of the chamber to a wafer susceptor is typically too small. Further, scaling up of the system is difficult.
An HDP source can be used for a Radical-Assisted Sequential (RAS) CVD process. The idea of RASCVD is similar to Atomic Layer Deposition (ALD) in which two precursors are supplied time-divisionally to the substrate. RASCVD differs from ALD, however, in that one of the precursors is a radical but not a stable compound. This method results in a monolayer controllable deposition with perfect thickness uniformity. However, if one of the precursors, namely the stable compound, has a low sticking probability, this process is not very effective.
In an effort to solve the above-described problems, it is a feature of an embodiment of the present invention to provide a high-density plasma apparatus, which can form a uniform radial distribution of radicals emanating from a plasma source.
It is another feature of an embodiment of the present invention to provide a high-density plasma apparatus which is capable of eliminating sputtering of an inductor and preventing back streaming of gas products near the inductor.
Accordingly, to provide the above features, according to an embodiment of the present invention, there is provided an inductively coupled plasma apparatus preferably including a process chamber, a top plasma source chamber, a reactor, an inductor, an opening, and a shutter. The process chamber has a wafer susceptor on which a substrate is installed. The top plasma source chamber is preferably installed on the process chamber. The reactor, which is installed in the top plasma source chamber, preferably has a channel through which a gas flows and supplies plasma reaction products to the process chamber. The inductor, having two ends, is preferably installed between the top plasma source chamber and the reactor and is preferably wound around the reactor. The opening is preferably positioned within a circumferential space, in which the inductor is installed, between the reactor and the process chamber. The shutter is operable to open and close the opening.
Preferably, the reactor includes an inner cylinder, an outer cylinder, and an annular channel, wherein the outer cylinder surrounds the inner cylinder. The annular channel is positioned between the inner cylinder and the outer cylinder. It is preferable that a top of the annular channel is connected to a gas manifold outside the top plasma source chamber.
According to an embodiment of the present invention, a bottom of the inner cylinder preferably narrows so that a bottom of the annular channel between the inner cylinder and the outer cylinder is changed into a circular shape. Preferably, a gas distributing plate having a plurality of orifices is installed in the annular channel. More specifically, the gas distributing plate may include a plurality of gas distributing plates that are spaced apart from each other in the annular channel.
Preferably, the wafer susceptor is electrically floating and supported by a ceramic vacuum break in the process chamber.
It is also preferable that a purge inert gas is supplied to the circumferential space in which the inductor is installed. Additionally, it is preferable that a length of the inductor is equal to mxc3x97xc2xc wavelengths of an applied high frequency electromagnetic field, where m is an integer. It is preferable that high frequency power is supplied to a turn of the inductor between the ends of the inductor, and that the two ends of the inductor are either grounded or floating. Further, it is preferable that a high frequency electromagnetic field and low frequency electromagnetic field are pulsed (i.e., turned on and off) either periodically or according to a given sequence.
It is also preferable that a plurality of bipolar pulses of a DC voltage be applied to the substrate. Finally, it is preferable that the pulses of the electromagnetic field are synchronized with a series of discrete supplies of a first and a second gas, and whereby the first and the second gases are supplied sequentially to provide a modified radical-assisted sequential deposition process.